In CDMA communication, there is a so-called distance problem that a signal transmitted by a communication terminal at a short distance becomes strong interference with signals transmitted by other communication terminals, as a plurality of communication terminals use the same radio frequency band. There is transmission-power-control in order to solve the distance problem. In the transmission-power-control, a base station apparatus makes instructions so that received levels of signals transmitted from each communication terminal become a predetermined level.
And, in the transmission-power-control of downlink signals transmitted from the base station apparatus to communication terminals, the communication terminals measure received SIRs of pilot symbols, and send transmission-power-control instructions to the base station apparatus based on the measured results. As the base station apparatus controls the transmission power based on the above transmission-power-control instructions, and transmits signals, the communication terminals may receive signals with a desired received level.
A conventional radio communication apparatus will be described, using a document, “Study on a transmission-power-control method using following-up control of fluctuations in instantaneous values in DS-CDMA downlink channels” (The Institute of Electronics, Information and Communication Engineers, Technical Report AP96-148, EMCJ96-83, RCS96-162, MW96-188 (1997–02)). This document has disclosed a transmission-power-control method in the CDMA. Hereinafter, the disclosed contents will be described.
In the transmission-power-control, measurement of SIR representing received quality, and increase or decrease of the transmission power are cyclically performed every one slot (with a period of 0.62.5 ms). In this case, after a measured SIR and a target SIR are compared, an instruction to reduce the transmission power is sent to a base station apparatus (transmission side) when the measured value is larger, and an instruction to increase the transmission power is sent to the base station apparatus when the measured value is smaller. The base station apparatus increases or decreases the transmission power according to the above instructions.
And the base station apparatus performs outer-loop control, considering that the target SIR depends on the environments of communication terminals in order to obtain the required quality (FER: Frame Error Rate). Specifically, FER is measured in the first place, using data after decoding. After the measured FER and the target FER are compared every several frames, the target SIR is raised when the measured value is larger, and the target SIR is decreased when the measured value is smaller.
As described above, in the conventional technology, the transmission-power-control instructions are sent to the base station apparatus based on SIRs measured at the communication terminals, and, at the same time, the transmission power is controlled by changing the target SIR through the outer loop.
Here, a total-transmission-power-value which is a transmission power for communication of the base station apparatus with a plurality of communication terminals is required to be made equal to or smaller than a maximum transmission-power, as the maximum transmission-power of the base station apparatus may not be made equal to or larger than a predetermined value by restrictions in amplification circuits such as amplifiers.
However, in a conventional base station apparatus, there has been a case in which the transmission power of the base station apparatus is near the maximum total-transmission-power value, as the transmission power is increased in a simple manner in order to secure the communication quality when there is an instruction from a communication terminal to increase the transmission power, for example, due to deteriorated communication environments.
When the shortage of the transmission power is occurred, the communication quality is deteriorated by transmission at reduced transmission power. Moreover, amplifiers and so on of the base station apparatus are made in an unstable state as a system, when transmission is continued almost in a limited condition, as the requirements to increase the transmission power may not be met. Furthermore, there is a problem that a new call may not be accepted due to shortage in the transmission power.